The laboratory has continued to investigate four chimpanzees which were inoculated in 1987 with a killed human immunodeficiency virus type 1 (HIV-1) vaccine and subsequently challenged with live virus. While the vaccine failed to protect a vaccinated animal or a na[unreadable]ve control from challenge, the effect of the vaccine on two previously infected animals was that virus expression appeared to be suppressed. Between 1987 and present, quarterly P-24 antigen detection assays have proven negative in the previously infected chimpanzees while the vaccinated and na[unreadable]ve animals frequently proved P-24 antigen positive. Polymerase chain reaction (PCR) assays on DNA extracted from peripheral blood lymphocytes of the previously infected animals show negative or weakly positive signals compared to strong signals from the vaccinated and control animals. Circulating antibody levels of all of the animals remain high with gelatin particle agglutination titers of between 1:4,096 and 1:102,400, suggesting that antigenic stimulation continues despite negative virological findings. All the animals remained healthy and showed no evidence of T-cell depletion. On 22 July 1997 one of the previously infected chimpanzees, number A-3, died suddenly, apparently of a stroke. Post mortem tissues were taken and lymph nodes were tested for the presence of viral antigen with negative findings. Further investigations are underway.